Visiting any secured buildings, you probably won't be allowed in with a folding knife of any kind. Not sure about flashlights. Are visitors to the MOMA/Met screened these days?
If you get into a situation where you need to use any sort of weapon, you've already failed at situational awareness.
Staying alert served me just fine when I lived in
hell.
Carrying a ball-peen hammer with no explainable use for it is most likely going to be deemed illegal carry of a weapon if you manage to get searched and it's found. A flashlight should be okay.
Check out this thread where a NYC peace officer talks about arresting people for carrying a kubotan:
http://forums.officer.com/forums/archive/index.php/t-42554.html
That's what you're up against if you carry a weapon in NYC: well-intentioned law enforcement who think it's in your best interest to arrest you for carrying most weapons that can be used in self defense. According to Shield22, assaulting minors for swearing, and charging people for carrying a kubotan, will help reduce violent crime.
Check for yourself, but a small folder (I'd go with 3.5" or smaller, folder, non-assisted) and a flashlight should be fine. After some quick research, it appears to me that NYC legal pepper spray must have a warning label and first aid instructions mandated by NY law. If you have some other spray not sold in NY, it's possible that it's illegal because it lacks those labels.
An umbrella is not as sturdy but would have better reach than a flashlight.
Some people have been known to develop a leg or foot ailment, requiring them to use a
cane, before travelling to regions that forbid concealed carry. A strange phenomenon, but be alert for any unusual pain or difficulty walking... it could happen to you.
IANAL nor a legal expert.
